Recently, semiconductor light sources are utilized for backlight devices for display devices and other lighting applications. Semiconductor light sources include semiconductor laser diodes (LD's) and light-emitting diodes (LED's). The brightness of light emitted by a semiconductor light source depends on the magnitude of the drive current. Consequently, to allow semiconductor light sources to light on stably, semiconductor light sources are generally driven by a constant current (i.e. constant-current control). This constant-current control makes it possible to control the current applied to semiconductor light sources to be constant against various changes during the control (such as fluctuations in the power supply voltage and fluctuations in load).
FIG. 1 is a block diagram showing a configuration of a conventional semiconductor light source driving apparatus that is generally used to perform a constant-current control of a semiconductor light source.
Semiconductor light source driving apparatus 10 shown in FIG. 1 is a constant current driving circuit using a current control loop. Output current detecting circuit 14 that detects the current applied to this semiconductor light source 12, is provided at one end of single semiconductor light source 12 or at one end of a plurality of semiconductor light sources 12 connected in series. In order to let a constant current be applied to semiconductor light source 12, the output of output current detecting circuit 14 is sent to current comparing circuit 16 and is compared with a current command value from current command section 18. Output voltage controlling circuit 20 performs a pulse width control in voltage source 26 according to the comparison result in current comparing circuit 16. To be more specific, voltage source 26 is constituted by: power supply source 28 such as a battery; DC-DC converter 30 of a drop-switching or boost-switching scheme for performing a DC-DC conversion of direct current power from power supply source 28; and smoothing circuit 32 such as an LC (i.e. an inductor and capacitor). Output voltage controlling circuit 20 controls DC-DC converter 30 according to the comparison result in current comparing circuit 16. The output voltage of DC-DC converter 30 is converted into a desired DC voltage value in smoothing circuit 32 and is supplied to semiconductor light source 12. In this way, negative feedback closed loop current control CL1 is performed. Further, output voltage controlling circuit 20 is constituted by proportional gain circuit 22 and compensating circuit 24.
With negative feedback closed loop CL1 constituted in this way, when the value of the current that is applied to semiconductor light source 12 is greater than the desired current value, a pulse-shaped square wave voltage of a short on-period is supplied to the gate of the switching element in DC-DC converter 30, so that the smoothed voltage that is supplied to semiconductor light source 12 decreases and the current of semiconductor light source 12 decreases. By contrast with this, when the value of the current that is applied to semiconductor light source 12 is lower than the desired current value, a pulse-shaped square wave voltage of a long on-period is supplied to the gate of the switching element, so that the smoothed voltage that is supplied to semiconductor light source 12 increases and the current of semiconductor light source 12 increases. By means of such a negative feedback closed loop current control, a desirable constant current that makes the output value of output current detecting circuit 14 the same as the current command value, is applied to semiconductor light source 12, thereby creating a stable state in semiconductor light source 12.
However, in conventional semiconductor light source driving apparatus 10 shown in FIG. 1, when a fluctuation occurs in control loop CL1 due to voltage fluctuations between power supply source 28 and DC-DC converter 30, noise from outside and disturbance noise entering output current detecting circuit 14, the constant-current control becomes unstable. Therefore, there is a technical limit that the response speed (i.e. frequency characteristics) and gain of closed loop CL1 cannot be increased very much. Accordingly, conventional semiconductor light source driving apparatus 10 may be best employed for goods such as mobile telephones that can function well enough as a backlight device by applying a fixed constant current to semiconductor light source 12 on a regular basis. However, conventional semiconductor light source driving apparatus 10 may not be best employed for goods that change the desired constant current value frequently, for example, goods of fields requiring the function of adjusting light to change the brightness of the light source.
Therefore, Patent Literature 1 proposes semiconductor light source driving apparatus 40 shown in FIG. 2. This semiconductor light source driving apparatus 40 with an addition of control loop CL2, is designed to drive semiconductor light source 12 by a constant current and to reduce heat generated in the circuit element group including semiconductor light source 12 by optimizing the voltage that is supplied to semiconductor light source 12, thereby making a backlight device light stably while light is adjusted.
As a specific configuration, in this semiconductor light source driving apparatus 40, as shown in FIG. 2, DC-DC converter 30, output driving element 42, voltage comparing circuit 44 and output voltage controlling circuit 20 form first negative feedback closed loop CL1 for controlling the supply voltage, and output driving element 42 and constant current controlling circuit 46 form second negative feedback closed loop CL2 for controlling the constant current. Output driving element 42 is a transistor or FET, for example. Further, with this configuration, the voltage generated across resistor (R) 48 connected in series with semiconductor light source 12, is detected and used for the control loops of both negative feedback closed loops CL1 and CL2. The value of this voltage is proportional to the drive current of semiconductor light source 12, and, consequently, negative feedback closed loops CL1 and CL2 form a two-fold current control loop. Generally, when a feedback loop is two-fold, interference is produced between the loops and the operation of the loops becomes unstable. Therefore, this configuration sets the frequency response characteristics of one loop (i.e. closed loop CL1) to a one-twentieth of the frequency response characteristics of the other loop (i.e. closed loop CL2), to prevent interference between the loops.